Contrastive analysis of cooling performance between a high-level water collecting cooling tower and a typical cooling tower

A three-dimensional (3D) numerical model is established and validated for cooling performance optimization between a high-level water collecting natural draft wet cooling tower (HNDWCT) and a usual natural draft wet cooling tower (UNDWCT) under the actual operation condition at Wanzhou power plant, Chongqing, China. User defined functions (UDFs) of source terms are composed and loaded into the spray, fill and rain zones. Considering the conditions of impact on three kinds of corrugated fills (Double-oblique wave, Two-way wave and S wave) and four kinds of fill height (1.25 m, 1.5 m, 1.75 m and 2 m), numerical simulation of cooling performance are analysed. The results demonstrate that the S wave has the highest cooling efficiency in three fills for both towers, indicating that fill characteristics are crucial to cooling performance. Moreover, the cooling performance of the HNDWCT is far superior to that of the UNDWCT with fill height increases of 1.75 m and above, because the air mass flow rate in the fill zone of the HNDWCT improves more than that in the UNDWCT, as a result of the rain zone resistance declining sharply for the HNDWCT. In addition, the mass and heat transfer capacity of the HNDWCT is better in the tower centre zone than in the outer zone near the tower wall under a uniform fill layout. This behaviour is inverted for the UNDWCT, perhaps because the high-level collection devices play the role of flow guiding in the inner zone. Therefore, when non-uniform fill layout optimization is applied to the HNDWCT, the inner zone increases in height from 1.75 m to 2 m, the outer zone reduces in height from 1.75 m to 1.5 m, and the outlet water temperature declines approximately 0.4 K compared to that of the uniform layout.     

自然通风冷却塔加装十字挡风墙数值研究

基于CFD软件和自然通风湿式冷却塔相关理论,对喷淋区和雨区采用离散相模型计算,对填料区建立了基于Poppe理论的数值求解模型,并通过编译自定义源项函数,实现其在Fluent中的求解。在该模型基础上,分析了横向风速对冷却塔热力性能的影响;最新提出了在雨区加装改进的十字挡风墙的措施,并引入多孔跳跃模型实现其数值求解。计算结果表明:横向风对出塔水温影响很大,当风速为6 m/s时,出塔水温最高,比无风时高1.34℃;改进的十字挡风墙能有效提高冷却塔性能,最大能使出塔水温降低0.32℃,并且能明显降低水损失。     

火电机组湿式冷却塔加装导流板的数值研究

基于CFD软件和自然通风湿式冷却塔相关理论,对冷却塔喷淋区和雨区采用离散相模型计算,对填料区建立了基于Poppe理论的数值求解模型。模拟了无外界因素影响的情况下冷却塔的热力性能。结果表明:冷却塔中心区域空气的温度高湿度大,影响整体的冷却效果。通过在雨区外围加装导流板,使空气旋转上升,增加中心区域空气扰动,增强换热能力;同时对喷水面实行分区配水,减少中心区域淋水量,结果使出塔水温降低0.481℃。     

Investigation of Natural Draft Cooling Tower in China

ABSTRACT

The natural draft cooling tower has been widely used in thermal power plants because of its stable operation, lower maintenance cost, and smaller environmental impact. Hundreds of natural draft cooling towers have been built in China, including different types and various capacities of conventional cooling towers, seawater cooling towers, flue gas injection cooling towers, cooling towers with water collecting device, dry cooling towers, and others. A wide range of investigation for natural draft cooling towers was carried out for the first time by the Electric Power Planning & Engineering Institute in 2013 in China. Based on the collected data and summarized information, the statistical results and conclusive opinions are analyzed. The development trend of natural draft cooling tower technology is also explored in this paper. The investigation results are expected to be a reference for designing and operating natural draft cooling towers in thermal power plants.     

Effect of Cross Wind on Performances of Natural Draft Counterflow Cooling Tower Outlet

ABSTRACT

The cooling efficiency of the counterflow natural draft cooling tower could be reduced by a cross wind. Therefore, it is necessary to study the cross-wind impact for optimizing the design. In this paper, a three-dimensional numerical model is used to study the influence of the cross wind on the pressure loss coefficient at the outlet of the counterflow natural draft cooling tower. The results show that an obvious additional draft is formed when the cross-wind velocity is more than four times of the mean velocity at the fill section. A formula to quantitatively analyze the influence of the cross wind is given in this paper.     

A Numerical Analysis of the Local Anomalies in a Natural-Draft Cooling Tower

This paper presents experimental and numerical analyses of the aero-thermodynamic characteristics of a natural-draft cooling tower. The influence of local technical faults in the fill and rain region on the cooling tower's performance was estimated. Measurements of the air velocity and the temperature above the droplet eliminators showed a noticeable non-uniformity of both parameters. This is caused by a non-uniform airflow resistance and heat-transfer rate within the fill and rain region. Based on these measurements, a commercial CFD model was customized with additional relations describing the heat- and mass-transfer, as well as the airflow resistance in individual regions of the cooling tower. The results of a 3D numerical simulation of the cooling tower are the temperature and velocity distributions within the entire cooling tower. A comparison of nominal and actual cooling tower operation shows regions with unfavorable air temperatures or velocities. Thus, the inefficiently operating areas of the cooling tower's cross-section can be identified. These areas cause non-homogeneous aero-thermodynamic characteristics and have an influence on the integral characteristics of the cooling tower. A sample calculation of an actual cooling tower shows the usefulness of the method when it comes to improving the cooling tower's performance. The improvement can be achieved by modifying the fill resistance and water distribution.     

Solar enhanced natural draft dry cooling tower for geothermal power applications

This paper presents a new concept of hybrid cooling, named solar enhanced natural draft dry cooling tower (SENDDCT), in which solar collectors are added to traditional natural draft dry cooling towers to increase their performance. The purpose of using solar energy in this new cooling system is to increase the suction through the tower so that more air flow is achieved through the compact heat exchangers that cool condensers of a geothermal power plant. For the same size of the cooling tower, more air flow across the heat exchangers means more heat can be rejected by the system. The governing equations for the SENDDCT are similar to those of a conventional natural draft dry cooling tower except that solar heating is added after the heat exchanger bundles. Performance comparisons show that SENDDCT has substantial advantages over conventional natural draft dry cooling towers for geothermal power plants as well as standalone solar chimney power plants.     

CFD Two-Scale Model of a Wet Natural Draft Cooling Tower

A 2-D axisymmetric model of multiphase heat, mass, and momentum transfer phenomena in natural draft cooling tower is developed using a CFD code Fluent. The fill of the tower is modeled as a porous medium. The energy and mass sources in this zone are evaluated solving a separate 1-D model of mass and heat exchange. The spatial dependence of the sources is accounted for by dividing the fill into a set of vertical channels. The CFD solver produces boundary conditions for each channel, while the model of the channel exports the heat and mass sources to the CFD solver. To accelerate the calculations, an original technique known as the proper orthogonal decomposition (POD) is applied. This approach produces a reduced dimensionality model resulting in significant time economy and accuracy loss lower than 2%. The Euler-Euler multiphase model is used in the rain zone. The simulation results have been validated against experimental data coming from field measurements of a large industrial installation.     

Numerical study of the thermal–hydraulic performance of evaporative natural draft cooling towers

In this paper, the mathematical and physical models governing the flow, mass and heat energy of moist have been set up for an evaporative natural draft cooling tower. The models consider the effect of non-spherical shape of water drops on the flow, heat and mass transfer. Experimental data has been adopted to validate the numerical scheme. Average difference between the measured and the predicted outlet water temperature is 0.26°C. Distributions of the velocity components of the moist air, density, pressure, enthalpy and moisture content, the water temperature and its mass flux have been predicted. The simulation shows that some recirculation exits under the lower edge of the shell, where the air enthalpy, temperature, humidity and moisture content are higher, but the density is lower. The simulation also proves that the main transfer processes take place in the fill region where the percentage of latent heat transfer is predicted as 83%. However, about 90% of the heat energy is transferred via evaporation in the rain region although the total heat transfer rate there is very small compared to the fill region. Hourly performance of a natural draft cooling tower under the meteorological condition of Singapore has also been predicted.     

横向风对湿式冷却塔热力特性影响数值研究

基于CFD软件和湿冷塔相关理论,建立了基于Poppe理论的填料区数值求解模型,对喷淋区和雨区采用离散相模型计算.模拟了横向风对冷却塔热力特性和飘水损失的影响;提出了沿塔周设置回流槽的措施,以减少飘水损失.结果表明:横向风对出塔水温影响很大;无风时,出塔水温最低,随着风速的增加,出塔水温先升高后降低,并且,随着环境温度、湿度的不同,在横向风速为5m/s～7m/s间取得极大值.     

冷却塔外复杂流场的数值模拟与实验研究

对某能源中心楼顶的冷却塔组、烟囱、通风孔等散热设备及整个楼顶空间的流场与温度场进行了整体数值模拟,探讨了高温烟囱帽引起的排烟方向改变对冷却塔组进风处空气温度场及流场的影响,并进行了实验测试。结果表明,烟囱帽使排烟向下折转,导致冷却塔进风处下部位置空气温度显著升高;较低的烟囱烟气直排时,高温烟气会先抬升一段距离,然后受冷却塔进风处负压区吸引进入冷却塔内。     

Simulation of a cross-flow cooling tower performance

The new two-dimensional mathematical model of the performance of a cross-flow cooling tower is presented. Our model includes a positive feedback between aerodynamics of cooling tower and a rate of evaporative cooling. The self-consistent iterative algorithm of its solution is proposed and discussed. The simulation results, which included profiles of air temperature at the rain zone, are displayed. It is shown that the main parameter, affecting on the thermal efficiency of cross-flow cooling tower, is the average droplet radius. The range of change of final droplet temperatures is calculated.     

The Effect of Windbreak Walls on the Thermal Performance of Natural Draft Dry Cooling Towers

A three-dimensional study using the standard k-? turbulence model to simulate airflow in and around a natural draft dry cooling tower (NDDCT) has been conducted using a general-purpose CFD code. This investigation considered the location and the porosity of windbreak walls' structure on the NDDCT thermal performance. In addition, the effect of the windbreak walls on the thermal performance of two NDDCTs with different capacities has been investigated. Two parameters have been used to show the effect of the windbreak walls on the NDDCT thermal performance. At the reference heat exchanger temperature, the thermal effectiveness parameter has been employed. At the reference rejected heat from the NDDCT, the change in the cooling tower approach parameter has been employed. The results in this paper show an improvement in the NDDCT thermal performance due to the introduction of windbreak walls. Moreover, optimizing the location of the windbreak walls has been shown to have a more significant effect on the NDDCT thermal performance than the porosity of the walls. In addition, the effect of the windbreak walls on the thermal performance is similar for the two NDDCT with different capacities.     

湿式瘦高冷却塔淋水装置节能技术应用

主要叙述了一种基于FLUENT UDF方法设计湿式冷却塔淋水装置的节能技术,并介绍其应用于电厂冷却塔的情况。该节能技术的应用研究主要包括,环境侧风对超大型自然通风冷却塔热力性能的影响;塔群效应和塔群效应与环境侧风叠加等条件下,超大型逆流式自然通风冷却塔的热力计算方法;基于FLUENT UDF方法准确分析塔内每个区域的热力参数(喷头实际出流能力、填料实际淋水密度和对应区域的集水池平均水温)等工艺性能关键技术研究;冷却塔新材料(不同波形、不同片距)、新设备的研发制造;冷却塔环境保护关键技术研究等。     

Thermal optimization of a natural draft wet cooling tower

This study attempts to quantify the potential improvement in a natural draft wet cooling tower (NDWCT) performance that can be attained by optimizing the fill and water distribution profiles across the tower and to provide designers with the modelling tools for such an investigation. A simple two‐dimensional (2D) model is described, which allows rapid evaluation of NDWCT performance for use with an optimization procedure. This model has been coupled with an evolutionary optimization algorithm to determine the optimal fill shape and water distribution profile to maximize the cooling range of a typical NDWCT. The results are compared against a 2D axisymmetric numerical model. The extended 1D model is found to significantly reduce computational time compared with the numerical model, allowing a wide range of parameters to be tested rapidly with reasonable accuracy. The results show that the optimal layout differs significantly from a uniform profile, with both the water flow rate and the fill depth decreasing towards the centre of the tower where the air is warmer with reduced cooling potential. The overall improvement in the tower cooling range is very low under the design conditions tested, due largely to the highly coupled nature of the airflow and heat transfer in the tower. It is concluded that any design modifications of the type considered would need to be carefully optimized to have any possibility of improving performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Water Consumption Comparison Between a Natural Draft Wet Cooling Tower and a Natural Draft Hybrid Cooling Tower—An Annual Simulation for Luoyang Conditions

ABSTRACT

The Merkel method, the Poppe approach, and an empirical equation are employed in the developed computer program to predict the evaporation loss for the natural draft wet cooling tower (NDWCT). All the three models are validated with the experimental results. The calculation results in the NDWCT of a 300-MW power plant in Luoyang show that both the Merkel method and the empirical equation could not make the accurate evaporation loss prediction as the Poppe approach does. The annual water consumption and economic benefits for the power plant employed with the NDWCT and the natural draft hybrid cooling tower (NDHCT) are calculated by using the hourly weather condition. The results show that the power plant would save more than 95% of water but lose US$15,464,306 per year by replacing the NDWCT with the NDHCT.     

Experimental and thermal analysis of energy efficient novel design wet cooling towers

Waste heat is generally dissipated from process water to atmospheric air in cooling towers. In the present study, a novel design is used to extract more amount of heat without any additional energy input by incorporating secondary ambient air in an induced draft wet cooling tower. In addition, more fresh air is induced in the tower from the rain zone, which increases the effectiveness at any value of the water to air flow rate (L/G ratio). Moreover, tower characteristics, range, and evaporation loss were also increased due to the novel design. It is noteworthy that secondary fresh air increases effectiveness, heat rejection, and tower characteristics by 10.12%, 19.65%, and 26.11%, respectively, and decreases approach by 16.32% at 0.55 L/G ratio, 44°C inlet water temperature, 29.7°C dry bulb temperature, and 18.4°C inlet air wet bulb temperature.     

Evaporative cooling of water in a mechanical draft cooling tower

A new mathematical model of a mechanical draft cooling tower performance has been developed. The model represents a boundary-value problem for a system of ordinary differential equations, describing a change in the droplets velocity, its radii and temperature, and also a change in the temperature and density of the water vapor in a mist air in a cooling tower. The model describes available experimental data with an accuracy of about 3%. For the first time, our mathematical model takes into account the radii distribution function of water droplets.Simulation based on our model allows one to calculate contributions of various physical parameters on the processes of heat and mass transfer between water droplets and damp air, to take into account the cooling tower design parameters and the influence of atmospheric conditions on the thermal efficiency of the tower. The explanation of the influence of atmospheric pressure on the cooling tower performance has been obtained for the first time.It was shown that the average cube of the droplet radius practically determines thermal efficiency. The relative accuracy of well-defined monodisperse approximation is about several percent of heat efficiency of the cooling tower. A mathematical model of a control system of the mechanical draft cooling tower is suggested and numerically investigated. This control system permits one to optimize the performance of the mechanical draft cooling tower under changing atmospheric conditions.     

Solution of heat and mass transfer in counterflow wet-cooling tower fills

Model of heat and mass transfer in wet cooling tower fills is presented. The model consist of a set of four 1D ODEs describing the mass and energy conservation and kinetics with boundary conditions prescribed on opposite sides of the computational domain. Shooting technique with self adaptive Runge–Kutta step control is applied to solve the resulting model equations. The developed model is designed to be included in a large scale CFD calculations of a natural draft cooling tower where the fill is treated as a porous medium with prescribed distributions of mass and heat sources. Thus, the technique yields the spatial distributions of all flow parameters, specifically the heat and mass sources. Such distributions are not directly available in standard techniques such as Merkel, Poppe and e-NTU models of the fill where the temperature of the water is used as an independent variable. The method is validated against benchmark data available in the literature.     

超大型湿冷塔进风流场及导流装置数模研究及应用

基于Fluent软件,采用标准k-ε湍流模型进行应力封闭,对某工程塔内传热传质过程进行三维数值计算。计算分析了塔内外空气的速度场、温度场,建立了相关方程及气水两相间传热传质理论模型。结合工程实际情况,对冷却塔进风流场进行深入分析:1)导风板的存在降低了塔侧空气绕流流速,增大了冷却塔进风口流场的对称性,使塔内空气动力场的均匀程度增加;导风板安装高度和长度对冷却塔进风流场产生较大影响,以高11 m、长8 m导风板对# 1、# 2冷却塔性能的改善作用最大;2)导风板安装角度和块数对冷却塔进风流场产生一定影响,在导风板安装角度由0°至20°变化、在导风板安装块数由60块至90块变化时,两塔冷却性能变化影响较小。